1. Field of the Invention
The present general inventive concept relates to a paper feeding apparatus of an image forming apparatus (e.g., a laser beam printer, a copier, an ink-jet printer or a photo-printer), and in particular, to a paper feeding apparatus of an image forming apparatus having a lifter to lift a paper loading plate loaded with sheets of paper so that a top of the sheets of paper loaded on the paper loading plate comes in contact with a pick-up roller with a constant pressure. The present general inventive concept also relates to a method of controlling the paper feeding apparatus.
2. Description of the Related Art
In general, an image forming apparatus, such as a printer or a copier, is provided with a paper feeding apparatus loaded with and receiving a plurality of sheets of paper to sequentially feed the loaded sheets of paper into a body of the image forming apparatus.
FIG. 1 illustrates an example of a conventional paper feeding apparatus 10 of an image forming apparatus.
The paper feeding apparatus 10 comprises a cassette 12 removably attached to a side of a body 11 of the paper feeding apparatus 10 and being loaded with a plurality sheets of paper S, a pick-up roller assembly 13 having a pick-up roller 14 for picking up and feeding the sheets of paper S loaded in the cassette 12 into the body 11, a knock-up plate (paper loading plate) 21 rotatably installed within the cassette 12 to lift leading edges of the sheets of paper S toward the pick-up roller assembly 13, a lifter 15 for pivoting the paper loading plate 21 toward the pick-up roller 14 side of the pick-up roller assembly 13 so that an uppermost one of the sheets of paper S is positioned at a pick-up position where a top of the sheets of paper S comes in contact with the pick-up roller 14 with a constant pressure, and a position sensor unit 30 for sensing whether the uppermost one of the sheets of paper S is positioned at the pick-up position.
The paper loading plate 21 is pivotally supported on a hinge bracket 25 by a hinge axle 26.
The lifter 15 comprises a lifting plate 16 for lifting the paper loading plate 21, and a driving motor 17 for pivoting the lifting plate 16.
A fixing part 18 is formed at one end of the lifting plate 16, wherein the fixing part 18 is fixedly connected to a first end 20a of a power transmission shaft 20 (FIGS. 2A and 2B). The power transmission shaft 20 is pivotally supported in the cassette 12 and projected out of the cassette 12.
As illustrated in FIGS. 2A and 2B, the driving motor 17 comprises a driving axle 17a and a coupling 23 provided at an end of the driving axle 17a, wherein the coupling 23 engages a second end 20b of the power transmission shaft 20 when the cassette 12 is inserted into the body 11, and the coupling 23 transmits a driving force of the driving motor 17 to the lifting plate 16 through the power transmission shaft 20.
The position sensor unit 30 comprises a first sensing lever 34 projecting from an end of a body 13a of the pick-up roller assembly 13, and a first optical sensor 31 having a first light emitting part 32 and a first light receiving part 33, which are mounted on a printed circuit board array (PBA) 50 (FIG. 3) opposite to the first sensing lever 34.
As illustrated in FIGS. 2A and 2B, the paper feeding apparatus 10 further comprises a pick-up roller lifting member 40 which lowers the pick-up roller 14 of the pick-up roller assembly 13 toward the cassette 12 to a position represented by a two-dot chain line (FIG. 3) when the cassette 12 is inserted into the body 11, and separates the pick-up roller 14 of the pick-up roller assembly 13 from the cassette 12 to a position represented by a one-dot chain line (FIG. 3) when the cassette 12 is removed from the body 11.
The pick-up roller lifting member 40 comprises a lifting guide 41 arranged in the body 11 to be moved by the cassette 12 and provided with a guide surface 43 for guiding the body 13a of the pick-up roller assembly 13 to ascend or descend, a compression spring 45 arranged between a sub-frame 11a and the body 13a of the pick-up roller assembly 13 to elastically compress the body 13a of the pick-up roller assembly 13 so that the body 13a of the pick-up roller assembly 13 comes in contact with the guide surface 43 of the lifting guide 41, and a tension spring 44 interposed between the lifting guide 41 and the sub-frame 11a to return the lifting guide 41 to its original position, thereby lifting the pick-up roller assembly 13, when the cassette 12 is removed.
The pick-up roller lifting member 40 may employ a configuration in which the pick-up roller lifting member 40 is mounted in relation to the pick-up roller assembly 13 so that the pick-up roller assembly 13 is raised or lowered by the pick-up roller lifting member 40, instead of being mounted in relation to the body 11 so that the pick-up roller assembly 13 is raised or lowered by the cassette 12, as described above.
The following is a description of the operation of the conventional paper feeding apparatus 10 of the image forming apparatus, as described above.
If the cassette 12 loaded with the sheets of paper S is inserted into the body 11 as illustrated in FIG. 2A, a projecting step 42 of the lifting guide 41 is pushed in a direction indicated by an arrow A by a front end of the cassette 12.
As a result, the pick-up roller assembly 13 positioned above the cassette 12 as represented by the one-dot chain line in FIG. 3 descends in a direction indicated by arrow B along the guide surface 43 of the lifting guide 41 (FIG. 2B) to the position depicted by two-dot chain lines in FIG. 3 while being pivoted about a pivot axle 13b (FIG. 1) by the compression spring 45.
Thereafter, when the cassette 12 is completely inserted into the body 11, the pick-up roller assembly 13 is in a state in which a part of the pick-up roller 14 is positioned above the leading edges of the sheets of paper S in the cassette 12, and the second end 20b of the power transmission shaft 20 projecting from the front end of the cassette 12 engages the coupling 23 provided at the end of the driving axle 17a of the driving motor 17.
In this state, the driving motor 17 is driven to rotate the power transmission shaft 20, so that the lifting plate 16 is upwardly pivoted, thereby lifting the paper loading plate 21.
As the paper loading plate 21 is lifted up, the top of the sheets of paper S loaded on the paper loading plate 21 comes in contact with and upwardly push the pick-up roller 14 against the compression spring 45. Accordingly, the body 13a of the pick-up roller assembly 13 is upwardly pivoted about the pivot axle 13b. 
When the pick-up roller assembly 13 is pushed up and positioned where the top of the sheets of paper S comes in contact with the pick-up roller 14 with a constant pressure, the first sensing lever 34, which is disposed at an end of the body 13a of the pick-up roller assembly 13, is arranged between the first light emitting part 32 and the first light receiving part 33 of the first optical sensor 31. This position is represented by a solid line in FIG. 3. Accordingly, the first optical sensor 31 produces an “OFF” signal.
As the first optical sensor 31 produces the “OFF” signal, a controller (not shown) stops the driving motor 17 and drives the pick-up roller 14 using a driving source (not shown) connected to the pick-up roller 14 using a gear train (not shown) so that the sheets of paper S in contact with the pick-up roller 14, i.e., the papers loaded in the cassette 12, are sequentially fed into the body 11 one by one beginning with the uppermost one of the sheets of paper S.
The conventional paper feeding apparatus 10, as described above, typically employs a stepping motor or a DC motor as the driving motor 17 for rotating the lifting plate 16.
With the stepping motor, if a number of steps of the stepping motor are obtained and the number of steps for stopping the stepping motor after the first optical sensor 31 of the position sensor unit 30 is turned “OFF” has been previously set, the stepping motor is properly stopped after it rotates by the set number of steps from a point in time when the first optical sensor 31 is turned “OFF.” The stepping motor rotates the set number of steps regardless of a load on the paper loading plate 21, i.e., the quantity or weight of sheets of paper S loaded in the cassette 12. Therefore, although it may be advantageous that a rotating angle of the lifting plate 16 and the paper loading plate 21, i.e., the pick-up position of the sheets of paper S, can be properly controlled, a rotational ratio of the stepping motor is high, and construction and installation of the stepping motor is complicated, because the stepping motor requires a large installation space.
On the other hand, construction and installation of the DC motor is relatively simple, and an installation space can be minimized. Additionally, the price of the DC motor is inexpensive. However, a number of revolutions of the DC motor can vary depending on a quantity or weight of the sheets of paper S loaded on the paper loading plate 21.
More particularly, the weight of 500 sheets of A3 papers is about 4.9 kilograms (kg), the weight of one sheet of A5 paper is about 2 grams (g), and the weight of the paper loading plate 21 is about 200 g. Therefore, the DC motor lifts any weight in a range of about 200 g to about 5.2 kg with a constant velocity.
However, because the number of revolutions of the DC motor of low capacity and low price varies depending on the quantity or weight of the sheets of paper S, a time period required for stopping the DC motor after the first optical sensor 31 is turned “OFF” also varies depending on the weight. As a result, a rotating angle of the paper loading plate 21, i.e., the lifting height, will also vary according to the quantity or the weight of the sheets of paper S.
If a lifting height of the paper loading plate 21 varies as described above, a frictional pressure between the pick-up roller 14 and the paper loading plate 21 or the sheets of paper S will vary. These variations cause the conventional paper feeding apparatus 10 to be unreliable.
According to an experiment, with 500 sheets of A3 papers, the lifting height of the paper loading plate 21 was 72 mm, and the frictional force produced between the pick-up roller 14 and the paper loading paper loading plate 21 or the sheets of paper S was 100 gram-force (go), and with one sheet of A3 paper, the lifting height of the paper loading plate 21 was 73.5 millimeters (mm) and the frictional force produced was between the pick-up roller 14 and the paper loading plate 21 or the sheets of paper S was 160 go.
Because the frictional force directly affects a feeding force of the uppermost one of the sheets of paper S, the pick-up roller 14 may not be able to pick up the uppermost one of the sheets of paper S. In addition, other pick-up problems, such as jamming or overlapped feeding of the sheets of paper S can be caused if the frictional force produced is outside a previously set range.
To solve these problems, a method can be considered in which an encoder is provided with the DC motor to calculate a difference of velocity depending on the quantity or the weight of the sheets of paper S and the rotation of the DC motor. The quantity or the weight of the sheets of paper S is compensated for based on the calculation so that the DC motor is additionally rotated. However, this method has a problem in that the manufacturing costs are increased because it becomes necessary to provide the encoder.